The sliding filament theory explains how muscles contract. According to this theory, muscle fibers contain two main types of protein filaments: actin and myosin. During contraction, the myosin filaments pull on the actin filaments, causing them to slide past each other. This sliding action shortens the muscle fiber, leading to muscle contraction. The process is triggered by the release of calcium ions and the presence of ATP, which provides energy. When a muscle receives a signal from the nervous system, it initiates this interaction between the filaments, allowing for movement and force generation in the body.